A case-control study is a retrospective study that looks back in time to find the relative risk between a specific exposure and an outcome.

A control group of people, who do not have the disease is used for comparison.

The researchers reported on the Asia-Pacific Journal of Clinical Oncology that in northeast China, a decreased colorectal cancer risk was observed in both men and women, who have higher allium vegetables intake, which includes garlic, garlic stalks, leek, onion and spring onion.